1. Field of the Invention
The invention relates to a circuit including a band gap circuit furnishing a first reference voltage being temperature independent, an amplifier circuit connected to the band gap circuit for receiving and amplifying the first reference voltage and supplying a second reference voltage, a negative feedback network connected to the amplifier circuit for feeding back the second reference voltage to the amplifier circuit, a sensor circuit connected to the amplifier circuit temperature-determining circuit parts thermally coupled to the sensor circuit and generating a temperature dependent voltage within the sensor circuit, the sensor circuit comparing the second reference voltage with the temperature dependent voltage according to a given comparison cirterion, and means being activated by the sensor circuit if the comparison criterion is fulfilled for controlling the temperature-determining circuit parts.
2. Description of the Related Art
Electronic power components, particularly integrated power circuits, require temperature protection against unduly high temperatures in the event of insufficient cooling which can lead to the destruction of the component. Ordinarily, a baseemitter junction of a transistor which is biased with a temperature-independent voltage in the forward direction serves as a temperature sensor. The sensor is activated due to the negative temperature coefficient of its threshold voltage or the positive temperature coefficient of the base current beginning with a given temperature and it reduces the heat loss in the component to be protected. Due to the thermal coupling of the temperature sensor and the power component, a control loop is provided wherein the control of the power loss is customarily achieved by switching the supply voltage for the power component on and off.
With this analog control, so-called pulse-width control, low-frequency sawtooth oscillations with superimposed high-frequency oscillations occur. The low-frequency oscillations occur because the control circuit operates very slowly due to the thermal feedback to the temperature sensor, and the high-frequency oscillations are generated due to the switching transients of the control, i.e., of the transistor used as the temperature sensor.
Particularly in integrated audio amplifier circuits, the low-frequency sawtooth oscillations are troublesome because they are within audible range. The output is at one-half the supply voltage potential of a B output stage which is preferably used in integrated power amplifiers and is connected through an electrolyte capacitor with a high capacity to a loudspeaker at which large current amplitudes occur, that may damage the loudspeaker during the switching processes of the control. The high-frequency oscillations generally lead to shunt currents in the B output stages, the output transistors of which can absorb so much power there that so-called "hot spots" are formed which lead to a local secondary breakdown and destroy the transistor or transistors.